1. Field of the Invention
The invention relates to a method for processing steel slags and optionally iron carriers such as, e.g., electric furnace slags, coverter slags, dusts derived from steel production, mill scale or secondary metallurgical residues.
2. Prior Art
A method of the initially mentioned type is described, for instance, in WO 99/14381. In that known method, steel slags were reduced above in an iron bath along with iron carriers and, in particular, along with fine ore in order to thereby form environmentally safe slags. To this end, SiO2 for adjusting a beneficial slag basicity or Al2O3 was also added among others, wherein the addition of iron carriers such as, for instance, fine ores in the first place served to appropriately use such starting materials which are difficult to process and which are difficult to reduce by conventional methods, in order to enhance the economy of the method. Due to the relatively poor thermal conductivity of slags and a heat capacity that is 1.5 to 2 times larger than that of iron, the heat transfer and degree of postcombustion to be attained are of essential importance to the economy of such a method. In any event, relatively high energy expenditures will have to be taken into account regarding that type of processing of steel slags aimed to produce environmentally safe slags.
The invention aims to improve a method of the initially defined type with a view to enabling, simultaneously with the processing of slags, the recovery of valuable raw materials which will guarantee the economy of the method despite the elevated consumption of energy involved. To solve this object, the process according to the invention essentially consists in that the molten steel slags and iron carriers, respectively, are supplemented with chromium ores or chromium-containing and/or nickel-containing dusts in order to adjust a slag basicity of 1.2 to 1.6, whereby the bath temperature is maintained at above 1600xc2x0 C., in particular between 1650xc2x0 C. and 1800xc2x0 C., and a carbon-containing iron bath is provided or formed, and that the slag and the ferrochromium alloy formed are tapped separately. It is true that the use of chromium ores or chromium-containing and/or nickel-containing dusts basically involves the risk of an excessive chromium oxide content within the slag, which might again seriously jeopardize the environmental safety of the thus processed steel slags. Yet, by working at bath temperatures of above 1600xc2x0 C., it has surprisingly become feasible to reach final chromium contents in the slag of far below 500 ppm and, at the same time, transform chromium from such chromium ores nearly quantitatively into the iron bath used for reduction. The activity of the chromium within the iron bath is considerably reduced by the formation of chromium carbide, to which end an iron bath having an accordingly high carbon content must be provided or formed. In this context, the carbon content of the iron bath, as a rule, must be kept at above 3 wt.-% C in order to ensure that the desired chromium carbide formation will be effected so as to avoid reslagging of the reduced chromium oxide. Final chromium concentrations of a carbon-containing iron bath provided in such a manner may comprise up to 60 wt.-% chromium, thus enabling a high-grade metal regulus in the form of socalled ferrochromium carburxc3xa9, in which the portion of carbon may amount up to 9 wt.-%. Due to that high-grade metal regulus, the economy of such a mode of procedure will be safeguarded despite the high temperature level required, whereby the thus formed pre-alloy, which may be used in the special steel plant, covers the overall energy costs and production costs of the slags that have thus become environmentally safer.
What is essential to the process according to the invention is the high temperature level of above 1600xc2x0 C., which will be ensured by a suitable postcombustion only. In addition, an accordingly rapid mass transfer must be ensured. Such conditions advantageously can be observed in that the chromium ores or chromium-containing and/or nickel-containing dusts are top-blown onto the bath via a hot blast lance at particle sizes of below 4 mm, preferably 0.5 to 2 mm. By blowing in the chromium ores at such particle sizes by the aid of a jet of a suitable speed, high turbulences will form, thus subjecting the bath to intensive agitation. The large relative surface implied by the particle size and the high shearing forces acting between the individual phases and, in particular, between the gas phases, the chromium ores, the slag melt and the metal melt result in accordingly high energy and mass transfer rates. The temperature level required in a simple manner may be ensured in that hot blast at temperatures of between 1200xc2x0 and 1600xc2x0 C. is used.
In order to ensure an appropriate postcombustion, the hot blast advantageously is enriched with oxygen to an oxygen content of 25 to 40 wt.-%, early incipient solidifying of the slag by the progressing reduction procedure advantageously being feasible by mixing the steel slags with the chromium ore in a suitable mixing vessel. In such a mixing vessel, the respective basicity and the mixed slag temperature may be adjusted, whereby the slag mixture may be tapped at temperatures of, for instance, about 1750xc2x0 C. and supplied to the consecutively arranged reduction unit. At those high slag input temperatures prevailing within the reduction converter, oxygen enrichment of the hot air jet may even be obviated at all.
Advantageously, it is proceeded in a manner that the molten steel slag and the chromium ores are mixed in a slag ladle and are electrically heated using, in particular, graphite electrodes and applied onto the iron bath at temperatures of above 1700xc2x0 C., preferably above 1750xc2x0 C., whereupon the iron bath is carburized and hot blast is top-blown. The iron bath in this case is to be present in a low-carbon form refined by hot blast in order to reduce the initial reduction speed, with high bath temperatures being safeguarded at the same time. Departing from carbon contents of, for instance, 0.8 to 1.5 wt.-% C within the iron bath, the carbon content required for the quantitative separation of chromium will be adjusted only subsequently by carburizing the bath.
In order to attain a particularly high environmental safety of the treated steel slag, the basicity advantageously is adjusted to 1.4 to 1.55.
An accordingly reliable mixing of the bath is feasible in a particularly simple manner in that the hot blast with the chromium ores or chromium-containing and/or nickel-containing dusts is directed onto the bath at jet speeds of more than 500 m/s and, in particular, 700 m/s.
In the context of the method according to the invention, special steel slags are preferably used as steel slags, yet also LD slags, OBM slags and electric furnace slags as well as secondary metallurgical residues may be envisaged as charging materials. Chromium ores may at least partially be replaced with dusts highly loaded with chromium and nickel, the introduction of phosphorus having to be minimized in any event.
In the main, the method according to the invention, in which carbon contents of up to 9 wt.-% may be obtained within the bath, offers the advantage that any reslagging of chromium within the slag can be largely excluded, residual chromium oxide contents of less than 360 ppm having been observed within the slag at temperatures of 1650xc2x0 C.